1. Field of the Invention
The invention relates to an organic electro-luminescence element (organic EL element) and a producing method thereof, particularly an organic EL element that has a small increase in voltage at the constant-current driving time and a long life and a producing method thereof.
2. Description of the Background
In an organic EL element, an organic emitting layer is held between an anode and a cathode. The anode is a transparent electrode and formed of ITO and the like. The cathode is the opposite electrode and formed of Al and the like. Electrons and holes are injected into the organic emitting layer from the electrodes, respectively, and the electrons combine with the holes therein to excite organic emitting molecules. The exciting molecules return to the base state from the exiting state while discharging energy, which energy is taken out as light.
In such an organic EL element, the insufficient adhesion of the anode to the organic emitting layer laminated thereon causes dark spots, thereby reducing its luminous efficiency. For example, when an organic EL element is driven by a constant current, its driving voltage significantly increases with the passage of time and then its life becomes shorter. Further when an organic EL element is driven at high temperatures, its emission becomes ununiform and the luminous efficiency is further reduced.
Thus in order to improve the adhesion between an anode and an organic layer, it is suggested to interpose a buffer layer made of organic or inorganic compounds between the anode and organic layer.
For example, Japanese Patent Kokai No. 10-214683 shows an organic EL element with a 1 to 500 μm thick amorphous film made of a metal such as Au or Pt, metallic oxide such as MoOx, VOx, SnOx, InOx or BaOx, or a conjugate polymer to improve defective contact between an anode and organic layer.
Japanese Patent Kokai No. 3-105898 shows an organic EL element in which a hole transporting layer and electron transporting layer, which have been generally made of organic compounds, are made of an amorphous semiconductor (p or n type) with a thickness of about 500 Å to improve the emission properties. The amorphous semiconductor can be an excellent film.
Japanese Patent Kokai No. 9-63771 shows an organic EL element in which a 5 to 30 μm thick energy barrier layer is formed of a metallic oxide with a larger work function than ITO such as RuO, MoO or VO between an ITO anode and hole transporting layer.
However, the surface state of an anode layer has not been considered in the above organic EL elements and the adhesion between anode layer and organic layer has not been sufficiently improved. Therefore when the organic EL elements are driven by a constant current, the driving voltage increases and dark spots occurs. Further the organic EL elements cannot be made thin and the production time becomes longer, since the buffer layer has a thickness as thick as at least 1 μm.
Japanese Patent Kokai Nos. 9-3-24176 and 9-204985 show organic EL elements in which a hole injecting transporting material subjected to a terminal-processing by a silane coupling agent is used or an anode is subjected to a surface processing by a titanate coupling agent to improve the adhesion between the anode and organic layer.
However, in any one of the organic EL elements, the adhesion between the anode and organic layer has not been sufficiently improved and the emission properties rather disperse.
Japanese Patent Kokai No. 9-260062 shows an organic EL element in which a mixed layer with a resistance of 20 Ω·cm or less is formed of a mixture of ITO and inorganic semiconductor instead of a hole injection transporting material in a thickness of about 50 to 1000 Å to avoid direct contact of an ITO anode with an organic layer.
However, the surface state of an anode layer has not been considered in the above organic EL element and the anode layer does not sufficiently adhere to the mixed layer.
Japanese Patent Kokai No. 11-126689 shows a method for producing an organic EL element in which a substrate is subjected to inverse sputtering by RF sputtering before the formation of a transparent electrode film so that the maximum surface roughness of the substrate is smaller than 15 nm and the average roughness is smaller than 10 nm.
Further the document teaches that the preferred distance between the substrate and target is form 4 to 15 cm in this method, since an ordinary RF sputtering device is used.
However, the method of Japanese Patent Kokai No. 11-126689 does not examine the composition of surface of anode layer, although it is directed to the roughness in the surface of substrate. Thus, the adhesion between the anode and organic layer is not sufficiently improved.
In addition, the distance between the substrate and target is narrow, since an ordinary RF sputtering device is used. As a result, it is difficult to change the conditions of inverse sputtering.
Therefore the organic EL element produced by the method of Japanese Patent Kokai No. 11-126689 is difficult to suppress increase when driven by a constant current and cannot satisfy the practical performances in information display devices and display devices mounted in vehicles.
It has been found that the increase in driving voltage is caused by surface defects in the anode surface which reduces the electron injecting properties. The surface defects is a thin layer with a thickness of about 10 Å existing in the surface part of anode layer and has a different composition from the interior (bulk) of anode layer. The surface defects are inevitably formed during the producing steps of organic EL elements. Their formation seems to be caused by etching residuals in the steps of patterning and cleaning the surface of anode layer, water absorption to the surface of anode layer, the deposition of impurities such as Sn atoms which are contained in minute amounts in the bulk of anode layer, or the like.
The inventors have made efforts to attain the above object and found that the adhesion with an organic layer can be effectively improved by reducing surface defects by improving an anode layer itself, specifically associating the surface composition of anode with the interior composition, thereby making the present invention.
An object of the invention is to provide an organic EL element with a long life in which voltage increase is small at the constant-current driving time.
Another object of the invention is to provide a method for efficiently producing such an organic EL element with a long life.